remove unnecessary require

AS::Multibyte are no longer required by access and filters string
core extensions.

Author: lest

activesupport/lib/active_support/core_ext/string/access.rb

@@ -1,5 +1,3 @@
-require 'active_support/multibyte'
-
 class String
   # If you pass a single Fixnum, returns a substring of one character at that
   # position. The first character of the string is at position 0, the next at

activesupport/lib/active_support/core_ext/string/filters.rb

@@ -1,5 +1,3 @@
-require 'active_support/core_ext/string/multibyte'
-
 class String
   # Returns the string, first removing all whitespace on both ends of
   # the string, and then changing remaining consecutive whitespace